Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION ;~
TITLE OF THE INVENTION
:,......................................................................... ...... . :. .
Wear Resistant Rails Having Capability
of Preventing Propagation of Unstable Rupture ~ :
BACNGROUND OF THE INVENTION ;
... ~ .... .. .
This invention relates to a wear resistant high strength "
rail utilized at a curved portion of a rail road, and more
particularly a wear resistant high quality rail having a
capability of Preventing propagation of unstable rupture. ~
Rupture of a rail often results in a lar6e train .
accident. A~ong tbe causes of the rqpture are included a
shelling da~age caused by the contact between the rail and a
car wheel, cracks at the joints between rails, shatter cracks
: :, .
in the rail, large oxide contaminants and deep surface -:
15 defccts. ~hen the rail is used over~a long timc, various
cracks presentinR at YariOUs portions of the rail result in
. . .
transversal or lon8itudinal defects due to fati8ue. Such
defects propagate with time. ~inally, the defect reachs a specific -
rupture toughness value (for example K ,~i value specified by :
ASTH ~399). bs a consequence, a rapid unstable rupture
- ~ ~ occurs, thus breaking the-rail. In a derail accident causeing ~ -
;~ ~ a iarge number of victi~s, when an end Or a wear resistant
; alloy steel including cracks producted at the time of cutting -~
~ith gas was exa~iDed, it was found that unstable rupture
cracks induced by a shock load caused by the car ~lùeel
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13286~8 : ~
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propagate over a length of more than 10 meters in the longi-
tudinal direction in the web of a rail. Such cracks branch
to the head or bottom, thus resulting in an accidental
rupture.
In a prior art wear resistant rail utilized to prevent `- ` ;;
these defects, the wear resistance property of its head con-
~ .. ..
tacting with the car wheel is constructed to have a fine
c~
pearlite structure having a higher strength than an ordinary
rail.
Although such fine pearlite structure provides excel~
lent wear resistant property, it is brittle so that its res-
istance to the generation of unstable rupture is small. Por
~ .
this reason, when USiDg such rail is necessary to always
prevent generation of surface and inside defects where there
is a fear of generating such defects it is necessary to
- perlodically detect such defects by ultrasonic fault
locater before the unstable rupture occurs so as to detect
and re~ove defective portions.
Although various measures have been tried such as in-
. . .
crease in the resistance to generation of-unstable rupture
caused by fatigue defects generated at specific portions,
for example a web and a bottom and increase in the
percentage of defecting the defects, these measures are not
always effective. ~ In any case, it is necessary to always
take care generation of defects and Propagation of unstable
:~ : ,....... -
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1 3 2 8 ~ 4 8 - ~
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destractive cracks. Yet, such generation of defects and
propagation thereof can not be avoidable. -
SUMMARY OP THE INVENTION - ^
Accordingly, it is an object of this invention to ~-
5 provide a wear resistant rail having a capability of `: ` :
preventing propagation of unstable cracks.
Hore specific object of this invention is to provide a
steel rail having a fine peartite structure capable of
manifesting a high wear resistant property and a high
strength aDd can bé used at a curved portion of a railway
.: , -
road.
another obiect of this invention is to provide a rail : :
capable of preventing propagation of unstable cracks to the ~`
head and web portions of tho rail, thus preventing large
15 accldent.
According to this invention, there is provided wear
resistant rail containing 0.50 to 0.85 wt% of C, 0.10 to ~ :
1.0 wtX of Si, 0.50 to 1.50 wt% of nn, less thin 0.035 wt% ;~
of P, less than 0.035 wt% of S. 0.050 wt% of Al and balance
of iron and i~purities, a web of the rail having a tempered
bainite structures and a high toughness. and a head of the
rail having a fine pearlite structure of high strength. --
By containing C of higher than 0.5 wt%, the wear
reslstant property can be provided. When the content of Si - ;
is increased more than 0.10 wtX. and when Mn is increased
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to more than 0.50 wtX, the strength can be improved. When ~ -'
the content of C is selected to be less than 0.85 wt% and when ~ '
the content of Si is selected to be less than 1.00 wtX
decrease in tbe ductility can be prevented. '
When the content of P and S is selected to be 0.035 wt%
respectively, degradation of the ductitlity and toughness
can be prevented. When the content of Mn is selected to be ~'
... . .
Iess than 1.50 wt%, degradation of the weldability can be " '.
avoided. Furthermore, where Al is selected to be less than
- 10 0.050 wtZ, degradation of the fatigue resistant property can ~ -''
be prevented. ` ' '- ~--
Where the web has a high toughness te~pered bainite ' '
structurs or ~artensite structure or a ~ixed tempered bainite- ''
artensite structure, propa8ation of the unstable'rupture can be
effectively prevented. Purthernore, by causing the h~ad to
have a fine pearlite structure of hl8h strength, the wear '
resistant property of the rall can be incresed. ''-
....: . .. .
BRIEP DeSCRlP~ION OF THE DRAWINGS ~:
In the acco~panying draw~ings; ~ ' i""' '''
. ..:
'20 Fig. 1 is a cross-sectional view of a rail; ~- --
Fig. 2 is a perspective view showing a bending test '---'
plece of a rail for~ed with a slit;' ~'
- Pig. 3 is a partial sectional view of the'rail alon~ ~ '
::::
the-sli~t;
25~ Pig. 4 is a side view of the web showing horizontal
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1328g~8 ' ' ' '`~"' '
A .
",,~
breakage test piece; ~-
Fig. 5 is a perspective view of a rai1 showing a - ' :
portion from which the test piece was taken, and - ;
Fig, 6 is a perspective view showing the manner of `
testing the horizontal breakage of the web.
DESCRIPTION OF THE PREPeRRED EMBODlnENTS
It has been found that a transversal crack in the head
or a horizontal crack in the web caused by fatigue propagates
.
to cause a large failure. especially a crack created in the ~ ~
10 web propagates in the longit~dinal direction and branchs to the ~ ;
head and bottom and causes a 1arge failure such as break off of
. . - ;
the head. Consequently, it is important to give a capability ~-
, . ~ .
to the web that arrest a high speed propagation ofan unstable
destructive crack. ~We have uade many researchesfor i~parting
to the web a capabilltY of 8rresting the high.speed propagation
.~ . .-
of unstable destructive cracks by variously varying the
: . .:
composition and structure of the rail steel and found that a
metal structure consisting essentially of tempered bainite ~:
structure, tempered martensite structurc and a mixture there
20~ o~f uanifest most excellent capability,~and that these structure -
: . . . : -
are ~uch more excellent than pearlite structuré or te0pered
p~earlite structure utilizèd for the web of the wear resistant
ra~ e have prepared test rails, the heads thereof compos
- ing of fine pearlite structure baving a high strength and the
25~ webs thereof composing of te~pèred bainite structure tempered
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martensite structure or a mixed tempered martensite-bainite
structure and subiected the test rails to fracture tests. '
The result of tests showed that these test rails have greatly -' '
improved the capability of arresting
5 prcpagation of unstable destructive cracks, thus preventing '''
breàkage or fracture of the rai1s. '''
The chemical co~position of the rail was l}~ited for
,. .: .~
the following reasons. ~ '' -~;
C is an indispensable ele~ent for impar~ting a wear '''~
resistant property. With less than O.50 wtX of C (herein-~
after erely designated by %) wear is severe so that it is
, ~ ~ . . . , ::,. -
i-possible to obtain practical wear resistant steel. On the ~ '~' '''~'
otherhand, with~C of hi~her than 0.85 %. pro-éuiectoid
ce~entite is for~ed in t~he wetal structure, ~ ; ~ ~ ' '"'' ''
thereby degradin8 the ductility. Por this reason tbe ran8e
of~C was~ ited between O.50 X and 0.85 %. ~ '
Si is used for the'pùrpose o~ deoxidation and for ..
~ proving the s'trength, Thus, for killed steel at least
'~ ~ 0.10 X of Si is necessary as a deoxdation ele-ent. Although
d ~ 20~th~e strongth can be improved with a larger~quantity ~f Si,
addition of~Si ty ~ore tban 1.00 X decreases the
d~uctll~ity~ For this re-soD 1.00 X is the upper li~it. '
n is~indisPeisabl~e for-i~pr-ovi~ng the~strength. With . '~ '
less than 0.50 Z, its effect of i~proving the strength is
-25 s~all, whereas ~ore than~l.50 X of ~n greatly degrades the - '"
:'~ " ' !' . ' .' ' . ' ' ' ''; '-
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weldability. Por this reason, the range of Mn was limited
to be 0.50 % to 1.50 %.
P and S are impurity ele~ents and when their content ~;
exceeds 0.035 %, both ductility and toughness degrade. For `
5 this reason, the upper limit of each of P and S is 0.035 X. ~ -
Al is used in combination with Si to act as a deoxidation
ele~ent. ~ith ~ore than 0.050 % of Al, not only a large .-
qualtity of Al203 is for~ed but also fatigue characteristic
de~rades, so that 0.050 X is the upper limit of Al. The ;'
10 corbination described above is essential for the rail of :
,~ .
this invention, and the ele~ents described above are ~ :
i~portant for for~ing fine pear1ite structure of high
strength in the head of the rail. Further~ore, these .:
ele~ents are necessary to ensure a ~inlmu~ hardenability
of the web.
It is necessary, one or eore of the following elements
are incorporated to steel for thc purpose of more
effectively ~anufacturing the rail of this invention. ~
Cr: 0.05 to 1.50 X ~o: 0.05 to 0.20 % -
~ V; 0.03 to 0.10 Z Nl: 0.10 tp 1.00 %
-; Nb: 0.005 to 0.050 X
:
These percentages we~e selected for the follobing
reasons~
- More particularly, Cr i~proves the hardenability,
. .
25~ thus making it ready to for~ the head as fine
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pearlite structure. ~oreover Cr increases the resistance to
softening of the pearlite structure at the time of tempering,
thus making it easy to obtain a fine pearlite structure of
high strength. Where the web is composed of bainite or
martensite structure, tne improvement of the hardenability
is effective to suppress admixture of the pearlite
structure necessary for transferring a pearlite nosc to a
long time side. Accordingly, 0.05 % that is necessary to
~: .
iqprove the heat hardening property is the lower limit, and
1.50 % is selected as the upper limit because Cr of higher
than 1.5 % degrades the weldability. For this reason, the
range of Cr was limited to be 0.05 to 1.50 X.
In the same manner as Cr, ~o increases the hardenability
. . .
and the strength because ~o increases the
re9istance to softenin8 due to heat tempering of the pearlite
structure. Thus, the ran~e of ~o is limited for the same
reason as for Cr. In other words, 0.05 % is necessary as
the lower limit for improving the hardenability
while 0.20 X is limited as the upper limit in view of the ~ -
weldability.
V and ~b not only improve the hardenability
but also manifest precipitation hardenin~, thereby ~-
- increasing the strength. As the oinimum quantities ~ ~ -
necessary for manifesting precipitation hardening, 0.03 % of
25 V and 0.005 X of Nb are necessary. -The upper limits of V ~- -
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13286~8
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and Nb are the quantities at which the effects of ~ and Nb ;
saturate, that is V: 0.10 % and Nb:0.05 %.
Ni is effective to improve the hardenability and ~:
. . .
to increase the hardness and toughness. With less than
.. ..: :
0.10 % of Ni. the hardenability is small while
with higher than 1.00 % of Ni, its effect saturates. For
this reason. the range of Ni was selected to be from 0.10 X
, . :.: .
to 1.00 %.
The rail steel having the chemical composotion
10 described above was heat treated under the following -
conditions for the purpose of obtaining metal structures -~
characterizing the invention.
~ore particularly. rolling heat is used, or if desired, ~-
a heat preserving furnace is provided or a}ter rolling and -
colling the rail it is rebeated to a temperature higher than
AC3 point. The Portion of the rail at a temperature higher
than the ACa point is cooled. Thus the head of the rail is
subjected to a slack quenching to form fine pearlite
structure having a high strength. The web is rapidly quenched~
20 to cool the shorter time side than the pearlite nose and ~ -
the cooling condition was changed for obta~ning a desired
aetal structure. To obtain the bainite structure, the rail
is 0aintained at a constant temperature between a
temperature higher thao ~s oint and a temperature lower
than Bs point ( upper limit teaperature necessary for
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forming the bainite structure ) for sufficiently proceeding
transformation. To obtain the martensite structure, the
rail is cooled to a temperature near a room temperature at any
cooling speed. If desired. marquenching process can be
S used. To obtain mixed structure of bainite and martensite,
suitable quantity of martensite is formed by cooling the
rail at a temperature lower than nS point and a desired
quantity of bainite is formed by heating the rail at a
temperature higher than Ms point but lower than Bs point.
Since the quantity of martensite formed depends upon the
temperature, a quantity of martensite is controled by under~~
transformation cooling rate from ~s point first. It is possible `
to control first the quantity of bainite by means of changing the
holding time at a transformation temperature. ~-
The web having bainite structure, martensite structure
or mixture of bainite and martensite whicb are formed as above
described is subjected to a heat treatment
followed by continuous tempering. Alternatively, the web is
coo1ed to room temperature and then tempered to obtain a
20 metal structure of high toughness. ~ -
When the web is heat treated, the joint region between
:: :
the web and the bottom would have a structure similar to that
,., ~.
of the web and it is inevitable to contain less than 30 % of ~-
pearlite structure in these structure described above. Even
25 ~hen the head and web are heat treated simultaneously or ~-~
independently, desired metal structures can be obtained.
Although there is no limit on ~ --
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13286~8 :::
the structure of the bottom, it is advantageous that the web
and the bottom have the same structure, usually pearlite
structure.
The method of preparing the rail of this invention can
S be applied to the web of an ordinary rail where a metal ~ ~ -
structure of high thoughness is desired. ;~
The following Table I illustrates the chemical composi-
tion of steel utilized in this invention.
-
~'
- ,
~, "
':, '- ,,
,~ '- ;~"' .
.
1~286~8 :
- 12 -
--, d ~=
s l ~ 3 o ~ o ~s
~ o ~o o o : . ,'
,." o ~ Y3 _
, r- d c,o, d
's ~ ~
~ 11~ 1
.' _ __ _
' '. i'J~ el: ~ ,~'i i~, :'~ ",''.' '
s~ ~ , ~ "' ~'`'`;'`' " '':'
. , . - . . ' ,-' ~ ' :',' ,:
1328~8 : ~`
..
.
- 13 -
Steel a is manufactured by the following methods 1 - 3. .
1. The rail of this invention wherein the head has a fine
pearlite structure of high strength and the web has tempered
bainite structure is manufactured as follows. The head is
5 cooled to a temperature lower than 500 c from a temperature -:~
higher than Ac3 point at a rate of 2-10 'c/sec. At the same
time ths web is quenched at a rate higher than 15 C~sec, :
and held at a constant temperature between 300 to 450 C.
When at least 50% has transformed into bainite, the web is .~
reheated to 600 - 700 c at a heating rate of more than :
1 'c/sec. After tempering the web is cooled, the bottom is
cooled naturally. ~ :
: 2. A rail of this invention having a head com~osed of fine : ..
pearlite structure and a web composed of tempered martensite :
. ,~ .
structure is manufactured as follows. The head is cooled
to a te~perature lower than 500 c from a temperature higher .-.
than Ac3 point at a rate of 2-10 c/sec. At the same time,
the wob is cooled to a temperature below ~s point (240'C), :~
that i9 a temperature (below 200 c) at which more than 50% is
20 transformed into artensite structure. If desired, at a ~ ...
. . .
- temperature immediately above Ms point, the coolin8 is
switched to weak cooling for effecting marquenchin8. ~:
; After that, the web is continuously heated to 600 - 700 'c -.
: at a rate of ~ore than l'C/sec, tempered and cooled.
.The bottom is cooled naturally.
3. A rail of this invention having a head composed of a
~ fine. pearlite structure and a web composed of tempered . . :~
.~ -, . .... .
.,'''.'',,'`~'' ','.'',,''' '','','"'." ''''`'.,''''"'-:~
. . . . : .
~. ,' ' ' ' " ' :
1328648 ~ ~ ~
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- 14 -
mixture of bainite and martensite structures is manufactured
as follows. Thus, the head is cooled to a temperature below
500 '~ from a temperature higher than Ac3 point at a rate of
2 - lO'C/sec. At the same time, the web is quenched at a
rate of higher than 15 c/sec and then held at a constant
temperature of between 250'C and 450'C. When more than 30%
has been transformed into the bainite structure, the web is
cooled to a temperature below ~s point, thus effecting
martensite transformation. Alternatively, the web may be
quenched to a temperature (200 - 100-C) at which more than
30% transformed into ihe ~artensite structure and thereafter
the web is continuously heated to 300 - 450 'c and held at this
temperature for effecting bainite tranformation. After form-
.
ing a ixture of bainite and martensite, the web is
15 continuously heated to 600 - 700~ . to effect tempering and ~
then cooled.Tbe bottom i9 cooled naturally. ~`
Where Cr, ~o, V, Ni, and Nb are incorporated as in the steel `~
species B - D shown ln Table 1, due to the improvement of
the hardenability it becomes possible to . ~ ~ -
: ~: ::
decrease the cooling rate of slack quenching the head and ~ ;;
of quenching the web In addit~on, the strength can be -
increased as above described.
After heat treating a rai! having a cross-sectional ~ -;
configuration of 136Re by the methods similar to -- -
. . : .
~ the ~ethods 1 to 3 of steel A, the
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unstable destructive crack propagation arresting capability
of the rail was evaluated according to the following
methods (i) and (ii).
(i) ~ethod of bending test of a rail formed with a slit.
By using a saw a slit 16 having a depth of 30mm and a
width of 3mm was formed across the head 12 of a rail as shown in
Figs. 2 and 3. The length of the rail is l.Sm. Then the rail
was mounted on supports spaced by 1000mm with the head 12
faced downwardly. and the slit 16 positioned at ;the center ;
between the supports. Then a static bending force is applied
to the rail to generate and propagate unstable destructive
crack at the slit 16. The propagation arresting performance
is iudged whether the rail breaks or not and the rail not
broken was judged as having the propagation arresting
perfor~ance.
(ii) ~ethod of web borizontal breakage tost,
Por the purposs of evaluating the arresting performance ;
of the hori20ntal crack of the rail xeb a test piece 7 as --
.... : :
shown in Fig. 4 was cut out fro~ a portion shown in Fig. 5.
The test was ~ade according to AST~ test ~ethod for crack
arrect tracture toughness to obtain a K, value.
This testing condition is shown in
Fig. 6. Thus the test piece 7 is mounted on a base plate 8
and a wedge 9 including a split pin 9a was driven into an -
.
-~ 25 opening 8 of the test Piece 7.
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- 16 - .
The test piece 7 has a thickness of 16mm. a width of :.
128mm, the width of slit 4 communicated with the opening 8
of lOmm, the length of the slit 4 from the opening 8 of 45mm ::
and the diameter of opening 8 of 25.5mm. ~:
When the value of K. value determined by the web
horizontal breakage test, (crack arresting toughness Yalue)is . ,:
higher than 200N8/mm3'2 it is judged that the test piece has
the unstable destructive crack propagation arresting ; ~ ~ .
performance. ~-
The test results of the rails of this invention and
prior art are shown in the following Table ~ which shows :`;
that the rails of this invention have much higher unstable
crack propagation arrestin~ performance than the prior art
wear resistant rails.
'`' "' ~ ~
'. ''~.i','
:'.. ': ,:'
'"'.-'
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,''"'-''.
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- 17 1 3 2 8 6 48
~ ~ r ¦ _ 0 ~ ~ ¦ 0 ~ ¦ A ~ ~ _ 1~ ¦ _ _
C~ l 3:9~ O ~ (1~ 11~ 2 ~ ~ q~ ~ O~ O ~3
._ _ A :~ A r r --_ ~ _ r-- : ~
A= I~ ~ r ~ z g3 a~ r~ o o~ ~ ~ z ~ o ~ ~
r ~ _ __ _ _ _ ~
~O ~ ~ ~ O ~ ~ ~ ~ O ~ O
O ~ r r l r : ~:
~ O cL ~ ~ ~ cL ~ ~ ,~ c~ ~ c~ ,~
D ~ I r l I ~
~ c.~ c~ r- u~ c~ a~ o c~ c~ o co r- , ,, ," ' .'
~ ~ ~ ~ ~ ~ ~ ~ cc~ c~ ~ ~o c~o
~ rr _ rl r l _r l l : :
~ ~ eL. CL CL . CL CL CL CL CL CL CL CL . ~,. .
. r_ q~ ~ ~ ._
¦ _ r ¦ 2 ¦ e ~ ~
~ _ ~ = = D r ----¦~ c ~ ~
I e e _ e L _ L_ L_ o c I : -:
f A - -:
1328~8 :~
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- 18 -
Remarks
1. p : pearlite structure
f p : fine pearlite structur~
T B : tempered bainite structure
T M : tempered martensite structure ' :~
T BM: mixed tempered bainite-martensite structure I ::
Some of web ~etal structure contains less than 30X of
pearlite structure. .
1 0 . ' ' . ' ' `,`.
2. The tempering temperature of the heb is 650'C.
. .
As above described according to this invention, in a rail .~
having fine pearlite structure of high strength and wear ; : .
resistant property capability of preventing propagation -~.
lS of unstable destructive crack fro~ the head to the web ~ .-
and unstable rupture propagating in the horizontal direc-
tlon through the web is l~parted to the web so as to . .
prevent breakage or large fracture of the rail. :. .
: ~:.:,, ,.:.,:
,,
.,
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~" , ;.~. :,,
,.
~ , : .,, ., , . ... - . ., , , -- , .. , . , . . ,, ... . , . ~ , . . . . . . . .. ...
